There are a number of commercially available systems for the removal of heavy metals from waste water streams. These systems are widely used in the plating industry and work fairly well where the heavy metal is present as free ions, as is usually the case in the waste from plating operations. Where complexes of heavy metals are involved however, as found in photographic processing waste effluents for example, the conventional metal removal systems are substantially ineffective.
In photographic processing waste, the heavy metal ions are tightly bound by complexing agents such as ethylenediaminetetraacetate (EDTA), nitrilotriacetic acid (NTA), and thiosulfate. These very strong complexing agents must be broken before the metallic ions can be removed from the waste solution.
Among the approaches shown in the prior art for the treatment of photographic processing waste containing heavy metal complexes, Bober et al U.S. Pat. No. 3,767,572 discloses a method wherein waste photographic processing solutions containing EDTA complexes are treated by chlorination of the waste solution under alkaline conditions to destroy the EDTA.
It has also been proposed to employ ozone in the treatment of photographic processing wastes. An article in the August, 1975 issue of Journal of Water Pollution Control Federation entitled "Ozonation of Photographic Processing Wastes" explores the usefulness of ozone in treating various kinds of photograhic processing wastes, including heavy metal complexes.
However, the waste treatment methods which are presently known have been found to be ineffective, or at best, of limited usefulness for the treatment of waste photographic processing waste in large quantities on a commercial scale.
For example, investigations conducted by applicants in work leading up to the present invention indicated that while ozone had the ability to oxidize and destroy the complexing agents found in photographic processing waste, the quantity of ozone and time of treatment required to achieve effective results were very high, thus rendering the use of ozone uneconomical and unfeasable for treating photographic processing waste on a commercial scale.
With the foregoing in mind, it is a primary object of this invention to provide a practical and efficient method and apparatus for treating a waste solution containing complexes of heavy metals to facilitate separation and recovery of the heavy metals and to reduce pollution of streams by the heavy metals.
A more specific object of this invention is to provide a treatment method and apparatus which may be economically applied to the commercial scale treatment of photographic processing wastes containing complexes of heavy metals such as silver, iron, cadmium and lead to permit recovery of the valuable metal content and to reduce pollution of streams from the heavy metals.